Bronze, bronze powders, and method of making the same



y 8, 1935. E. J. HALL 2,002,891

BRONZE, BRONZE POWDERS, AND METHOD OF MAKING THE SAME Filed June 4, 1931INVENTOR I BY W ATTORNEYS Patented May 28, 1935 UNITED STATES PATENTOFFICE BRONZE, BRONZE POWDERS, AND METHOD OF MAKING THE SAME ApplicationJune 4, 1931, Serial No. 542,189

Claims. (Cl. 134-76) ments of metal in the form of flakes or plates orscales. This flake form is desirable, in order that a small weight ofmetal may be made to cover the maximum surface and produce a metal-likecoating when applied as a paint. The flake form of the particle is,however, only one important feature of a desirable bronze powder.Another is the requirement that when the powder is applied as a paint,the flakes shall lie parallel to the surface. How completely this resultis attained depends upon the characteristics of the flake surface, andto some extent, upon the character of the medium or vehicle in which thepowder is dispersed. Considering effective bronze powders now known tothe trade, it is believed that the surfaces of the flakes are providedwith a film which gives the so-called leafing. or mirroring property andcauses some of the particles to float on the surface of the vehicle inthe same manner as a greasy needle will float onthe surface of water.

In current practice the metal'in pieces of convenient size, to beconverted to bronze powder, is placed in a stamp mill with a smallfraction of a per cent of some lubricating material such as tallow,olive oil, etc., and pounded or stamped.

As the particles are suflicientlyreduced in size they are eithercontinuously or periodically removed by screening or a current of gas.This may entail several repeated operations. Following the stamping andsizing operation the powder is placed in a drum which is provided withpolishing brushes. These brushes are caused to travel over the innersurface of the drum, giving a polishing action and cause to bedistributed over the surface of the particles stearic acid which isadded with the powder or subsequently. After polishing fora number ofhours varying with conditions, the powder is then stored and allowed toage. In this aging operation certain changes take place which enhancethe leafing property.

I have discovered that stearic acid or its equivalent, when dissolved ina suitable liquid can be made to impart expeditiously the desiredleafing property. And, further, that this property can be developedcoincidentally with the grinding or flaking operation. It has furtherdeveloped that several factors are important in the accomplishment ofthis result. For instance, if the combined operation of flaking and filmdevelopment is to be carried out the grinding must be by impact and notattrition, and the metal must not be overworked. That is, there is anoptimum force or blow at which the metal particle should be struck andas the force increases beyond that point results become progressivelyless satisfactory. The temperature at which the operation is carried onmust be controlled, because here again an optimum condition exists.There must also be a relation between the amount of metal, the size ofmetal particle, the amount of stearic acid and aluminum stearate, theamount and kind of liquid, atmosphere and temperature. Theseaforementioned factors are not capable of calculation and therefore mustbe empirically arrived at.

I have found that the leafing property can be imparted to a clean flakedmetal powder by treating it with the leafing agent in solution in asuitable liquid, in the presence of an oxygen available atmosphere, andthat the action is accelerated by heating. As an example: A given weightof flaked metal is heated with agitation in the presence of air to C.with five times its weight of a 2% solution of stearic acid in benzeneand the excess liquid then removed. The resulting product will leaf inbenzene.

As the leafing and flaking operations can be more conveniently carriedon simultaneously I prefer to so operate. While the comminution can becarried on in various grinding devices that operate by impact I preferto utilize a ball mill.

In the drawing, Figs. 1 and 2 illustrate a ball mill such as may be usedin the practice of my invention.

. Fig. 1 illustrates the proper operation of the mill, as hereinafterset forth, so that the balls act on the metal particles largely byimpact; and

Fig. 2 shows What happens when the proper operating conditions do notobtain, and there is too much of attrition and rubbing action.

Per 1 pound of 100 mesh comminuted aluminum (see paragraphs A and B) ithas been found that 1 pound of a petroleum fraction oil, such as"Varnolene (see paragraph C), 3% of stearic.

acid (see paragraphs D and E), 1% of aluminum stearate (see paragraph E)and 20 pounds of balls (see paragraphs F and J) give good results whenground for seven hours (see paragraph G) in a mill 16" diameter x 6"(see paragraph H) using A. Aluminum is used as an illustration becauseit is the bronze powder in greatest demand and most representative. Thesame principles, however, apply to other metals and alloys.

B. A IOO-mesh is specified because it has been found convenient todisintegrate the metal to this degree of fineness before starting theflaking and leafing development operation.

0. A petroleum fraction such as Varnolene is selected as the grindingliquid because it is a common constituent of varnish, further, itsboiling range is such that there is no excessive loss by evaporation orgreat fire hazard, yet the liquid can readily be removed by evaporationwhen a dry product is desired. Liquids such asturpentine, benzol, ornaphtha may be substituted for the Varnolene, as the liquid in which theleaf or mirror-producing film is developed. It

does not necessarily follow, however, that the particles will leaf inthis liquid. For instance, if the aluminum is ground in Varnolene itwill leaf in that liquid, and also in the paint vehicle. If aluminum isground in naphtha it will not leaf in the naphtha but will leaf in thepaint vehicle. The quantity used is adjusted to give the desiredemulsion. (See paragraph E).

D. There is distinct evidence of chemical reaction between the metal andthe stearic acid, in

which oxygen plays a part. As grinding proceeds the amount of stearicacid in solution decreases and a compound, probably aluminum stearateappears. If the grinding is carried on in a tightly closed mill therewill be a drop in pressure, provided the temperature rise is notsuflicient to give a compensating vapor pressure. Further, if anoxygen-free gas, such as hydrogen, is used a drop in pressure does notoccur and the leafing or mirroring action does not develop. Thefllmproducing agent must have an aflinity for the metal being treatedand develop a film which has a certain deportment with the vehicle ofapplication, the exact nature of which is not definitely known. Additionof the agent may be made at of the operation or toward the end. Whilestearic acid is given as the film-producing agent,,such substances aspalmitic acid may be employed. Particularly where the leafing film is tobe produced on the metal during the flaking operation, an excess ofstearic acid in the sludge as herein set forth seems to give a very goodresult. This may be for the reason that stearate dislodged from theflakes as by attrition during the milling operation does not re-attachitself, but the leafing film is repaired or restored by reaction of someof the excess stearic acid with the denuded flake. 3% of stearic acidseems to provide a sufllcient excess or reserve, though manifestly theproportion may be varied.

E. In the operation of a ball mill such as illustrated there aredifferent zones of action (see J) and it isnecessary to have the metalacted upon so far as possible in the impact zone. To prevent the metalfrom filtering through the interstitial of the ball mass and appearinglargely in the attrition zone, it is necessary to produce an emulsion orfroth carrying the metal particles which has sufficient body andtenacity to coat the balls and travel with them through the impact zone,yet permit suflicient mobility of the particles that they may presenttheir flattened sides to adjacent balls and mill sides on contact.Stearic acid and aluminum stearate are indicated for this purpose,though any number of substances may be employed, such as palm oil, oleicacid, olive oil, palmitic acid, etc. The froth producing agent may alsohave the function of imparting the leafing film to the metal duringgrinding, e. g., stearic or palmltic acid; or it may subserve only thefroth producing function, e. g., palm oil, etc. In the latter case anadditional agent will be employed to impart the leafing film, and thefroth producing agent will be selected and/or controlled as to amount soas to hinder as little as possible the development of the leafing film.

F. A ratio of 20 to 1 of balls to aluminum has proven satisfactory forthe efiicient grinding with the production of a brilliant flake and goodleafing quality.

G. The grinding time required is influenced by practically all the otherfactors mentioned and fineness of product desired.

H. The length and diameter of mill determine its capacity and output andthe selection of size is a matter of economics.

1. For the given set-up balls were used, but it is to be understood thatthe force required is dependent upon the physical properties of themetal and the size of the pieces being ground, and that this willinfluence either or both the ball size and the mill diameter. It is tobe further understood that the specific gravity of the metal beingtreated has a bearing on the ratio of metal to other factors. If it isdesired to treat scrap foil or sheet directly they are first cut, intopieces approximately 4; to /2 inch square and for foil 2/1000 to 4/1000of an inch thick grinding can be carried on to advantage in twostages-first, balls in a 16" mill and finished as above; second, forsheet 2 to 3" balls can be used to advantage'for the initial stage.

J. The speed of a ball mill determines whether grinding is done byattrition (Fig. 2), or by impact largely (Fig. 1). When the mill isrevolved the balls are carried up the side with a tendency to slippage.Thisslippage is retarded by the insertion of internal ribs. When acertain height is reached, see Fig. 1, dependent on the charge, andbelow certain speeds, the balls in the upper layers roll'down while thelayers of balls adjacent to mill wall are being carried up with theballs revolving on unoriented axes. The points in contact have a rubbingmotion and therefore an abrading action which, in the lower layers, isintensified by the weight of superincumbent balls. This is not asatisfactory flaking action but results in the production of anexcessive amount of slimes, as well as being destructive of the leafingfilm. The effect is less pronounced with a decrease in the size of ballsand an increase in the number of points of contact. By carrying theproper charge of balls in the mill, about twofifths full, and revolvingat the proper speed the balls by a combination of centrifugal force andgravity, are carried up to a point where they are thrown clear of themill shell and ascending balls and fall in a free path striking theopposite side of the mill and balls that have preceded them, with theproduction of desired flakes. From the point of impact, in thedirection'of rotation,.there is a zone of attrition but the majorreduction in particle size is by impact. For the most efiicient grindingmetal particles must lie between the impacting surfaces, and this isaccomplished by the aforementioned froth or emulsion. It is also patentthat the size of the ball and diameter of p the mill determine the forceof impact. Too heavy a blow is destructive of the flake and. the filmsurface and these two factors, with the resistance of themetalparticles, determine the ratio of mill diameter to ball size.

ethanolamine stearate is desirable.

K. To properly charge a 16" diameter x 6" mill, 2 lbs. of aluminum andlbs. of balls are required for the desired height. Reducing the amountof metal increases the fineness of product, while conversely increasingthe metal increases the coarseness of product, other things being thesame.

L. 50 C. is adopted as a satisfactory temperature for operation. As thetemperature rises above 50 C. there is a progressive tendency to loss ofleafingf and when at 100 C. it is seriously reduced. V

The material as taken from the mill, with the removal of a part of theliquid, is a finished paste product without further treatment and can bestored indefinitely without losing its mirroring property. It is anestablished fact that ready mixed bronze powder paints lose theirbrilliancy on standing and therefore it is customary to market thepowder and vehicle separately and mix on the job. This has severalinconveniences due to the extremely light character of the powder.Therefore, I prefer to market my product in paste form and for thatpurpose an emulsifying agent such as aluminum stearate or tri- Ingrinding with stearic acid in solution there is apparently aluminumstearate formed and this formation increases with temperature and byproper control a satisfactory paste can be produced. However, I preferto add aluminum stearate to the charge as it increases grindingefiiciency as well.

A typical product in paste form contains 58% aluminum; 1% stearic acid;1% aluminum stearate, which as stated helps the grinding action and alsohelps in the production and maintenance of the homogeneous paste form;and 40% of a petroleum fraction, such as Varnolene. The flakes of theproduct possess the leafing quality,

.manifestable in ordinary paint liquids, and include no ingredientswhich will impair the leafing quality.

If a dry product is desired, the liquid can be evaporated undercarefully controlled temperature and atmospheric conditions and theleafing property will be maintained. It is safe to heat the paste to 50C. in an oxygen containing atmosphere; if the temperature goes muchabove there is a tendency to loss of leafing property, and, if heateduntil thoroughly dried, at 100 C.,

. the leafing property is more or less completely destroyed in thepresence of oxygen.

In a non-oxidizing atmosphere such as city gas it is permissible to drythe paste at 200 C. and above without destroying the leafing property.It is possible to use a vacuum in drying, but this lowers thepermissible temperature as compared to drying in a gas atmosphere.

Because of the composition of the liquid, there will be no undesirableresidue on evaporation. Thus if the product in its paste form should heaccidentally subjected to evaporation there would be no dried paint filmskin such as would make aluminum, copper, zinc and tin, with no reasonto doubt a similar behavior with their alloys.

Where particular materials are mentioned in the claims, it is to beunderstood that equivalentmaterials may be substituted, except where thelanguage of a claim clearly excludes equivalents.

And where in,a claim it is stated that a certain procedure, involvingcertain materials, temperatures, etc., is followed, substantially as setforth, reference is implied to typical examples given in thespecification, not as limitative of the claim, but as illustrating thetechnique which constitutes the real invention-empirically determinedcontrol and regulation of various factors, each in view of the other.

Varnolene, referred to herein, as a petroleum fraction oil with aboiling point from in the neighborhood of 307 F. to around 405 F. (SeeU. S. patent to this applicant, No. 1,569,484.)

I claim: I

1. The method which comprises imparting the quality of leafing ormirroring to flake metal powder by subjecting such powder to the actionof a leafing agent dissolved in a volatile paint thinner, the amount ofliquid being suflicient to form a sludge with the metal, and thereafterremoving excess liquid to leave the residue in the form of asubstantially homogeneous stable paste.

2. The method which comprises imparting the quality of leafing ormirroring to flake metal powder by subjecting such powder to the actionof a leafing agent dissolved in a volatile paint thinner with oxygenavailable, the amount of liquid being sufficient to form a sludge withthe metal, and thereafter removing excess liquid to leave the residue inthe form of a substantially homogeneous stable paste.

3. As a new article of manufacture, a stable and homogeneous pasteadapted for ready admixture with a paint vehicle, such as varnish,including flake bronze powder having on its surface a leafing film, theremainder of the paste comprising a paint thinner which is innocuous tothe leafing film.

4. The method of producing bronze powder from metal particles, whichcomprises charging a ball mill with balls and with a quantity of themetal particles and a quantity of liquid containing in solution aleafing agent of the kind and for the purpose set forth. which imparts aleafing film to the metal and forms a creamy sludge with the metal inthe operation of the mill, rotating the mill with oxygen available atsuch speed as to cause cataracting of the balls and to throw them clearof the mill shell and ascending balls, continuing the rotation until themetal has been brought to the desired fineness and flake form,maintaining in the mill a temperature favorable to the creation andmaintenance of the leafing film, removing the sludge from the mill, andreducing the proportion of liquid to metal to such an extent as toproduce a substantially homogeneous bronze powder paste.

5. A method which comprises imparting the quality of leafing ormirroring to flake aluminum powder by subjecting such powder to theaction of a leafing agent dissolved in a volatile paint thinner, and inthe presence of an oxygen containing atmosphere, forming a leafing filmcomprising aluminum stearate upon such particles and removing the liquidand preventing loss of the leafing film.

quality of leafing or mirroring to flake metal taining atmosphere andremoving the liquid in a non-oxidizing atmosphere at a temperature notsubstantially higher than about 200 0., whereby lossof the leafing filmis prevented.

8. The method of producing bronze powder from aluminum particles, whichcomprises charging a ball mill with balls and with a quantity of themetal particles and a quantity of liquid containing in solution aleafing agent of the kind and for the purpose set forth, which imparts aleafing film to the metal and forms a creamy sludge with the metal inthe operation of the mill, rotating the mill with oxygen available atsuch speed as to cause cataracting of the balls and to throw them clearof the mill shell and ascending balls, continuing the rotation until themetal has been brought to the desired fineness and flake form,maintaining in the mill a temperature favorable to the creation andmaintenance of the leafing film, removing the sludge from the mill, andseparating liquid constituents therefrom; the solution comprising aliquid having the'properties of- Varnolene" in which is dissolvedstearic acid, the Varnolene being in the proportion of one pound per onepound of aluminum, and the stearic acid being in the proportion of 3% ofthe weight of aluminum.

9. As a new article of manufacture,'a mixture of about 58% of flakedaluminum particles, about 1% stearic acid, about 1% aluminum stearate,and about 40% Varnolene, the flaked aluminum having a leafing film, andthe mixture being in 2, substantially homogeneous and persistent pasteorm.

10. As a new article of manufacture, a wet pigment for admixture with apaint vehicle for the purpose of forming a paint, comprising flakedmetal bronze powder, the powder flakes having on their surface a leafingfilm as evidenced by the ability of the flakes to leaf in a paintvehicle, such as varnish, and being dispersed in a liquid carrierinnocuous to the leafing film and which forms with the flakes a stablepaste in which the flakes retain their color and leafing quality duringstorage, as evidenced by the fact that the paste can be heated attemperatures up to about 50 C. without substantial impairment in thoserespects.

EVERETT J. HALL.

